We intend to use a new approach in identifying prostate tumor-specific antigens. Prostate carcinoma cells will be fused with normal cells and hybrids isolated in selective medium. From previous experience we expect the hybrids to be nontumorigenic in nude mice. Tumorigenic segregants will be isolated. Nontumorigenic (NT) and tumorigenic segregant (TS) hybrids will be used to immunize rabbits. The anti TS antiserum will be absorbed with NT cells and vice versa. We expect to find that the absorbed TS antiserum will continue to react with TS hybrid cells and prostate carcinoma cells but not with NT hybrid or normal cells, thereby identifying a tumor-specific marker. This absorbed antiserum will be used to purify the tumor-specific antigen (TSA) for further characterization and also to generate sufficient amounts of TSA to use as immunogen for monoclonal antibody production. The absorbed TS antiserum and the monoclonal antibodies against the prostate TSA will be screened against a large panel of cell lines and tissues in order to confirm the specificity of these regents.